CLHS: Function READ-CHAR-NO-HANG

#

Function READ-CHAR-NO-HANG

Syntax:

read-char-no-hang `&optional` input-stream eof-error-p eof-value recursive-p => char

Arguments and Values:

input-stream -- an input stream designator. The default is standard input.

eof-error-p---a generalized boolean. The default is true.

eof-value---an object. The default is nil.

recursive-p---a generalized boolean. The default is false.

char---a character or nil or the eof-value.

Description:

read-char-no-hang returns a character from input-stream if such a character is available. If no character is available, read-char-no-hang returns nil.

If recursive-p is true, this call is expected to be embedded in a higher-level call to read or a similar function used by the Lisp reader.

If an end of file[2] occurs and eof-error-p is false, eof-value is returned.

Examples:

    ;; This code assumes an implementation in which a newline is not
    ;; required to terminate input from the console.
     (defun test-it ()
       (unread-char (read-char))
       (list (read-char-no-hang)
             (read-char-no-hang)
             (read-char-no-hang)))
    =>  TEST-IT
    ;; Implementation A, where a Newline is not required to terminate
    ;; interactive input on the console.
     (test-it)
    >>  a
    =>  (#\a NIL NIL)
    ;; Implementation B, where a Newline is required to terminate
    ;; interactive input on the console, and where that Newline remains
    ;; on the input stream.
     (test-it)
    >>  a<NEWLINE>
    =>  (#\a #\Newline NIL)

Affected By:

*standard-input*, *terminal-io*.

Exceptional Situations:

If an end of file[2] occurs when eof-error-p is true, an error of type end-of-file is signaled .

See Also:

listen

Notes:

read-char-no-hang is exactly like read-char, except that if it would be necessary to wait in order to get a character (as from a keyboard), nil is immediately returned without waiting.

The following X3J13 cleanup issue, not part of the specification, applies to this section:

- ARGUMENTS-UNDERSPECIFIED:SPECIFY

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